Aircraft nose undercarriages are known that include a steerable bottom portion carrying the wheels thereof. The bottom portion is steered by a steering actuator that includes at least one driving actuator, e.g. a hydraulic actuator or an electric motor adapted to cause the steerable bottom portion to pivot in response to a steering order delivered by the pilot or a by computer of the aircraft, the order specifically representing an angular position.
In the particular circumstance of electromechanical steering actuators, it is known to control the motor of the steering actuator by means of servo-control having two feedback loops, specifically an outer loop controlling position and an inner loop controlling speed. Such servo-control is performed in conventional manner by using a proportional integral derivative (PID) controller, that outputs a current setpoint for the motor of the electromechanical actuator. Also in conventional manner, the motor is associated with a controller that servo-controls the motor to the setpoint current received from the PID controller.
As a general rule, position and speed signals are delivered to the computer that implements the position servo-control by means of wire connections. Nevertheless, ever increasing consideration is being given to using a communications bus extending down along the undercarriage for the purpose of connecting the computer (generally contained within the airframe of the aircraft) to the various actuators and sensors located on the undercarriage, for the purpose of collecting and delivering data and orders necessary for implementing steering control.
In particular, it is envisaged to make use of deterministic fieldbuses of the type having time-divided multiple access, e.g. time-triggered protocol (TTP) buses. Unfortunately, such buses present a sampling interval of the order of a few milliseconds, which leads to instabilities when they are used in conjunction with PID controllers for steering control. It appears that the instability is associated with the slowness of the speed information.
An intermediate possibility would be to move the computer closer to the actuator so as to enable an internal speed feedback loop to be implemented using wires, so as to avoid the slowness of communication over the deterministic bus. Nevertheless, that computer would then be situated in a zone that is exposed and that is subjected to high levels of stress (bad weather, spray, vibration), which would require a great deal of protection to be provided.